In the fabrication of fluorescent lamps, a phosphor layer is coated on the interior surface of a glass lamp envelope using a paint-like suspension of phosphor powder. Although the composition of the suspension varies from manufacturer-to-manufacturer, the composition usually includes, in addition to the phosphor, a film forming binder, solvent(s) for the binder, and if necessary surfactants, defoamers and wetting agents. Most non-phosphor components of the coating suspension interfere with efficient lamp operation and longevity, and must be removed by pyrolysis in a manufacturing step known as "lehring". Another inorganic, non-fluorescent, component of the coating suspension is a submicron particle sized material, such as silicic acid or alumina, which helps bind the phosphor to the glass after the "lehr" process has removed the organics. Butler in his book, Fluorescent Lamp Phosphors, Technology and Theory, Penn State University Press (1980), gives a rather complete description of lamp coating technology and its evolution from the nitrocellulose and ethylcellulose-type lacquers to the newer polymeric binders that employ water as the solvent in place of the environmentally objectionable and flammable organic compounds. Depending on the chemical composition of the phosphor and method of preparation, phosphors may exhibit some differences in performance depending on whether they are deposited from organic-based suspension or from water-based suspension systems. Usually these differences are not significant unless the phosphor has a tendency to react chemically with one of the suspension components.
A variety of technologies have been described recently, e.g., U.S. Pat. No. 4,585,673, that permit the coating of phosphor particles with a thin film of a refractory oxide. This film can be made from a choice of different refractory oxides. Some of these, particularly Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3, have been found effective in protecting the phosphor against processes that cause lumen depreciation in fluorescent lamps. One example of such a protective coating, is that described in U.S. Pat. No. 4,585,673, in which the phosphor particles were coated with Al.sub.2 O.sub.3 by pyrolizing an aluminum alkyl in a fluidized bed of phosphor powder. Experimental fluorescent lamps employing such coated phosphors, particularly those using Zn.sub.2 SiO.sub.4 :Mn (Willemite) have shown significant improvements in lumen maintenance relative to lamps employing uncoated phosphors. Lamp test data also indicate that Al.sub.2 O.sub.3 coated Zn.sub.2 SiO.sub.4 :Mn phosphors (ACPs) perform equally well with regard to luminance and lumen maintenance when applied to the lamp envelope from freshly prepared organic or water-based suspensions. If, however, the ACP is applied from a water-based suspension that has been held-over for several days before use, the beneficial effects associated with the oxide coating are lost. This presents an obstacle to the commercialization of lamps based on the coated phosphor technology. Government regulations have already eliminated the use of organic-based phosphor suspensions in many parts of the world, and reintroduction and/or expansion of this old technology is therefore not a viable option. Moreover, the use of only freshly prepared water-based phosphor suspensions is precluded by the short useful life of these suspensions and by the cost of dumping the large volumes of aged material that would be generated in a modern automated lamp manufacturing facility. Therefore, it is desirous to provide a method which will improve the useful life of water-based phosphor suspensions which presently ave poor hold-over characteristics.